Within the semiconductor industry, there are numerous applications that require bonding two or more semiconductor wafers together, an example being semiconductor sensors formed by a silicon wafer (referred to herein as a device wafer) with a micromachined structure or micromachine, which is capped by a second wafer (referred to herein as a capping wafer). Specific examples of semiconductor sensors include yaw (angular rate) sensors, accelerometers and pressure sensors, each of which typically entails a cavity formed in the capping wafer to receive and/or provide clearance for the micromachine of the device wafer. Absolute pressure sensors require that the cavity be evacuated and hermetically sealed, while the performance of yaw sensors and accelerometers with resonating and tunneling micromachines generally benefit if the cavity is evacuated so that the micromachine operates in a vacuum.
The integrity of the bond between the wafers is essential for promoting the life of a semiconductor sensing device. As a result, various bonding techniques have been suggested for the purpose of maximizing the strength and reliability of the bond. For example, the use of adhesives, dielectrics such as glass frit, and solders as intermediate bonding materials have all been suggested in the prior art. Silicon direct and anodic bonding techniques that do not require an intermediate material have also been used. As can be expected, each of these bonding techniques can be incompatible or less than ideal for certain applications. An example of particular interest here is the manufacture of resonating and tunneling micromachines that require a vacuum for improved performance. Silicon direct and anodic bonding methods require very smooth bonding surfaces, and therefore cannot produce a vacuum seal when unplanarized metal crossunders are employed, as is often required to electrically interconnect resonating and tunneling micromachines to bond pads outside the vacuum-sealed cavity of a sensor. In contrast, organic adhesives, glass frit and solder can be used to cover metal steps of up to 21,000 .ANG. found on CMOS, bipolar and BICMOS wafers. However, organic adhesives have not been found to reliably seal micromachines under vacuum, and bonding techniques employing glass frit require temperatures typically in the range of about 385.degree. C. to 410.degree. C., which can cause polysilicon, electroformed metal and LIGA micromachines to warp, bend and/or become electrically unstable. As a further example, yaw sensors with resonating micromachined structures are prone to exhibit zero offset drift, compass effect and start-up drift if subjected to the temperatures necessary to bond wafers with glass frit.
In contrast, solder wafer bonds can be formed at temperatures of 350.degree. C. and less, and have been successfully used to form vacuum seals between wafers having a micromachined structure, as disclosed in U.S. patent application Ser. No. 08/785,683 to Sparks et al., assigned to the assignee of the present invention. Because solder alloys cannot wet or bond to semiconductor materials such as silicon and ceramics, solder wafer bonding requires solderable bond pads adhered to each wafer and to which the solder will metallurgically bond. As shown in Sparks et al., solder bonding of two wafers to form an evacuated cavity requires a pair of complementary solderable rings on the device and capping wafers. A drawback to this requirement is the close alignment tolerances required to align the solderable rings, which complicates the bonding process when performed in a vacuum to obtain an evacuated cavity for housing the micromachine.
From the above, it can be appreciated that improved bonding processes are desired to form a semiconductor sensor having a micromachined structure enclosed in an evacuated cavity and metal crossunders interconnecting the micromachine to bond pads outside of the cavity.